Basically from talking to everyone here, like a lot of the other posters said, its just a matter of time until your bike is stolen. I think you CAN prevent it by a) using 2 or 3 GOOD locks... everyone here uses 2 or more locks, but they are cheap locks and chains you buy for 9 Euros at the center market. b) dont use the "lock your bike to itself" system that so many dutch people do. Yes, there is NO place to lock your bike in most areas because there are literally THOUSANDS of bikes around you at all times. In a lot of places you can see a bike that has probably sat locked in the same place for months (flat tires and rusting away with no seat and bent rims from being hit by cars). But walk a few extra feet to FIND an area to lock your bike up. My friends bikes have all been stolen when he was in a rush and didnt lock his bike TO anything, just used 2 locks and locked the bike to itself.

I would say, if you are touring through the country on your own bike, bring lots of locks and dont leave your bike out overnight, if you are just flying in and visiting, buy or rent a cheapo beater, the cobblestone roads will be a lot to handle on a rigid fixie and it was a really nice comfortable ride sitting on a tank with big fat slicks. oh yeah, HAVE FUN and report back on how it was!